In X-ray computed tomography apparatuses, the technique of switching a focus size of X-rays in accordance with a set tube current value has been used. As X-ray detection elements of an X-ray detector have been downsized, a high resolution mode or a super high resolution mode in which a reading channel pitch is shorter in the channel direction or in the row direction in comparison with a normal resolution mode has been realized. However, in the case where the focus size is switched in accordance with the set tube current value, the channel pitch is not taken into consideration, and accordingly, there may be a case where CT imaging cannot be performed with an optimal focus size for a designated resolution mode.